POLYMORPH AND POLYTYPE IDENTIFICATION FROM INDIVIDUAL MICA PARTICLES USING SELECTED AREA ELECTRON DIFFRACTION

Dioctahedral micas are composed of two tetrahedral sheets and one octahedral sheet to form TOT or 2:1 layers. These minerals are widespread and occur with structures differing by (1) the layer stacking mode (polytypes), (2) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (3) the charge-compensating interlayer cation and isomorphic substitutions. The purpose of the present study was to assess the potential of parallel-illumination electron diffraction (ED) to determine the polytype/polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns were calculated along several zone axes close to the c*- and c-axes using the kinematical approximation for trans- and cis-vacant varieties of the four common mica polytypes (1M, 2M1, 2M2, and 3T). When properly oriented, all ED patterns have similar geometry, but differ by their intensity distribution over hk reflections of the zero-order Laue zone. Differences are enhanced for ED patterns calculated along the [001] zone axis. Identification criteria were proposed for polytype/polymorph identification, based on the qualitative distribution of bright and weak reflections. A database of ED patterns calculated along other zone axes was provided in case the optimum [001] orientation could not be found. Various polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered.